1. Field of the Invention
The present invention relates to a drill bit, in particular to a drill bit for a hammer drill for drilling in stone, concrete and the like, and including a drilling head formed of a hard material body with radial legs and having at least three radially extending cutting bits formed on the radial legs of the hard material body, and a centering tip provided in a center of the hard material body and projecting beyond one of the cutting bits, with the centering tip having linearly extending auxiliary cutting edges and limiting surfaces stretching between the auxiliary cutting edges.
2. Description of Prior Art
The drilling head of a drill bit is generally formed of a hard material body that is obliquely secured on the end surface of a drill stem. Alternatively, instead of forming the entire drilling head of a hard material, only the working ends of the drill bits are formed of a hard material as hard material inserts which are inserted in respective recesses formed in the drilling head. The drilling head is secured to the drill stem, e.g., by welding or soldering. The hard material inserts can be formed as a one-piece element having a predetermined shape or be formed of several separate elements which, when assembled together, have the predetermined shape. Usually, as a hard material, a hard metal or a sintered material is used.
The known drill bits are provided with centering tips which provide for punctual centering at the start of the drilling process, preventing skewing of a bore during large bore depths. In order to insure a proper guidance of the centering tip during drilling, the centering tip is usually noticeably axially exposed relative to the drilling head. When the drill bit is used for drilling in a reinforced concrete, often, as a result of the axial extension of the centering tip, the drill bit strikes a reinforcing iron, which leads to a breakup in the region of the centering pin. This sharply reduces the performance capability of the drill bit.
International Publication WO 01/08840 A1, discloses a drill bit with a centering tip formed by concave cutting edges of the cutting bit of the drilling head. The drawback of this drill bit consists in that the excavation rate of the drill bit is reduced in comparison with a drill bit having linear cutting edges.
German Publication DE 100 53 344 A1 discloses a drill bit having three radially extending cutting bits which are formed on radial legs of a hard material body. In the center of the body, there is provided a centering tip that projects beyond the cutting bits and is formed of flat limiting surface which stretch between linearly extending auxiliary cutting edges.
French Publication FR 2 779 366 A1 discloses a drill bit with a drilling head having three radially extending bits which are formed on radial legs of a hard material body in the center of which a centering tip is provided that projects beyond the cutting bits. As in the German Publication DE 100 53 344 A1, the tip is formed by flat limiting surfaces stretching between linearly extending auxiliary cutting edges.
A drawback of all the known drill bits consists in that the material strength in the region of the centering tip is noticeably reduced because of the pyramidal structure defined by flat limiting surfaces. As a result, the drilling speed is also reduced. Further, as a rule, the number of linearly extending, auxiliary cutting edges, which are formed by the limiting surfaces of the centering pin, is limited to the number of major cutting bits.
Accordingly, an object of the present invention is to provide a drill bit in which the foregoing drawbacks are eliminated.
A further object of the present invention is to provide a drill bit with a particularly stable centering tip and having a good excavation rate. a good excavation rate.